1. Field of the Invention
The present invention relates to liquid crystal display devices and, more particularly, to liquid crystal display devices employing a nematic liquid crystal and a polymer orientation film.
2. Related Art
The conventional display modes of display devices using liquid crystals include dynamic scattering (DS) type, twisted nematic (TN) type, electrically controlled birefringence (ECB) type, phase change (PC) type, memory type, guest-host (GH) type, surface stabilized ferroelectric (SSF) type, Heilmayer type and Whitetyler type. These modes differ depending on the method of converting electric signals applied to the liquid crystal into optical information.
Among these, TN type and its improvement, super twisted nematic (STN) type, both mainly employing a nematic liquid crystal, are currently used for commercial display device articles such as watches, hand-held calculators, word processors, personal computers, television sets and the like. These two types of display devices utilize the optical rotatory power of a liquid crystal.
Another type of known liquid crystal display devices is a dye blend type which utilizes light absorption by a dichroic dye blended in a liquid crystal. Heilmayer-type and Whitetyler-type display devices have been proposed as dye blend type display devices. Heilmayer type utilizes the combination of a liquid crystal blended with a dichroic dye and a single polarizer plate. Whitetyler type utilizes a chiral dopant which allows molecules of a liquid crystal blended with a dichroic dye to be helically oriented between substrates, thereby requiring no polarizer plate.
However, since the operational principle of TN-type, STN-type, Heilmayer-type and Whitetyler-type liquid crystal display devices is based on the field effect which utilizes the dielectric anisotropy of liquid crystal molecules, the response time obtained is on the order of several hundreds to several dozens milliseconds. Accordingly, the conventional combinations of nematic liquid crystals and display mode devices cannot provide a response speed fast enough for applications such as to a CAD terminal which require faster response. In addition, their electro-optical effect is attributed to the switching between two states of liquid crystal molecules, i.e. a state wherein the liquid crystal molecules are in a homogeneously twisted orientation and a state wherein the liquid crystal molecules are upstanding with respect to the plane of the substrate. Hence, the viewing angle dependence on the twist direction of liquid crystal molecules is unavoidable due to the operational principle.
As a liquid crystal display device which provides faster response, N. A. Clark and Lagerwall have proposed a surface stabilized ferroelectric liquid crystal display (SSFLCD) in Appl. Phy. Lett., 36,899 (1980), Japanese Unexamined Patent Application No. 56-107216 (1981), and U.S. Pat. No. 4,367,924. The SSFLCD utilizes electrical interaction between a polarity generated by the spontaneous polarization of a smectic liquid crystal and a polarity generated by an electric field to achieve the switching on cones on which liquid crystal molecules are movable. This allows significantly faster switching than the nematic liquid crystal, and offers an advantage of no viewing angle dependence. However, there still exist some problems. That is, the layer structure of smectic liquid crystal molecules makes the orientation control difficult, the orientation once broken by a shock is hardly recoverable, and a like problem.
Furthermore, two types of bistable liquid crystal display devices using a nematic liquid crystal have been proposed by Georges, Durand.
One of such types of liquid crystal display devices uses chiral ions to generate drive torques, as disclosed in PCT Publication No. WO 91/11747. This display device uses both dextrorotatory and levoratatory chiral ions blended in a liquid crystal to provide ununiform ion distribution for the generation of drive torques by applying voltage. Like SSFLCD, the application of pulsed electric field switches the liquid crystal molecules in parallel relation to the plane of the substrate. However, the use of impurity ions in this device presents critical problems in its reliability.
The other type utilizes flexoelectrical polarization caused by orientation distortion to generate drive torques, as disclosed in PCT Publication No. WO 92/00546. This device is free from any serious problems due to impurities, and is more reliable. Like SSFLCD, the application of pulsed electric field switches the liquid crystal molecules parallel to the plane of the substrate, and the response time of this device is about 100 .mu.sec. There is no viewing angle dependence because liquid crystal molecules are switched parallel to the plane of the substrate. The use of a nematic liquid crystal eliminates such problems as presented by SSFLCD, i.e. difficulty in orientation control and lower shock resistance, and enjoys a wider range of operational temperatures.
In FIG. 10, there is shown the structure of a nematic bistable display device utilizing flexoelectrical polarization which was reported by Georges, Durand in SID Precedings 606-607 (1991) and Appl. Phys. Lett., 1085-1086, Vol. 60 No. 9 (1992). FIG. 10 illustrates the display as including glass plates 7 and 8, liquid crystal layer 9, transparent electrodes 10, SiO orientation films 11 and spacers 12. The SiO orientation film is deposited by evaporation at an angle of 74.degree. with respect to the substrate normal as having a thickness of 30 .ANG., and the diameter of the spacers is about 1 to about 3 .mu.m. This condition stabilizes the orientation C of a liquid crystal molecule perpendicular to the direction of SiO evaporation and parallel to the plane of the substrate, as shown in FIG. 11. However, when twist power is generated by the addition of a chiral dopant, two orientations A and B appear wherein the liquid crystal molecule is tilted by .theta..degree. with respect to the plane of the substrate with its projection onto the plane of the substrate being deviated by .alpha..degree. and --.alpha..degree. from the direction of SiO evaporation. That is because the anchoring energy between the interface and the liquid crystal molecules is low.
FIG. 12 shows a direction of SiO evaporation and directions in which the liquid crystal molecules can be stably oriented. The directions of the orientation treatment (or SiO evaporation) on the upper and lower substrates are twisted by 45.degree. from the anti-parallel relation. The liquid crystal material contains a chiral dopant that allows a single liquid crystal body to be twisted by 22.5.degree. between the upper and lower substrates when a orientation of the single liquid crystal body is not controlled. The twist direction of the liquid crystal orientation is opposite to the twist between the SiO evaporation directions on the upper and lower substrates as shown in FIG. 12. The liquid crystal material injected under this condition have stable molecular orientations restricted under the effect of the chiral dopant, which includes two stable combinations of 1 to 3' orientation and 3 to 2' orientation.
FIGS. 13a and 13b are cross-sectional views of the liquid crystal display device, which correspond to the 1 to 3' orientation and 3 to 2' orientation, respectively, as shown in FIG. 12. If the molecule of the liquid crystal used in the device is of wedge shape, flexoelectrical polarization is generated by splay-type orientation distortion. The directions of the flexoelectrical polarizations are shown by the arrows in FIGS. 13a and 13b. The vertical components of the flexoelectrical polarizations shown in FIGS. 13a and 13b have opposite directions. Therefore, the bistable switching between these two states shown in FIGS. 13a and 13b is achieved by reversing the direction of the vertical component of the flexoelectrical polarization by the application of pulsed electric field.
However, the conventional bistable nematic liquid crystal display device employs an SiO oblique evaporation film as an orientation film and it is therefore difficult to form a uniform film over a large area. This will result in a lower productivity.
The above mentioned display device, when employed as a liquid crystal display device of dye blend type which uses a dichroic dye blended in liquid crystal material, has a low optical efficiency because the rotation angle of the longitudinal molecular axis of liquid crystal is about 45.degree..